1. Field of the Invention
The invention provides a device for transmitting torque to a rotary drill bit when drilling a borehole in subsurface formations. The device is for use in drilling systems where the drill bit is mounted on a drill collar in a bottom hole assembly mounted on a drill string which is rotated at the surface, the rotation being transmitted to the drill bit through the drill string and drill collar.
2. Description of Related Art
The invention is particularly, but not exclusively, applicable to rotary drag-type drill bits of the kind comprising a bit body having a shank for connection to a drill collar on the drill string, a plurality of cutters mounted on the bit body, and means for supplying drilling fluid to the surface of the bit body to cool and clean the cutters and to carry cuttings to the surface. In one common form of bit some or all of the cutters are preform (PDC) cutters each comprising a tablet, usually circular or part circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate, which is usually of cemented tungsten carbide.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations or soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters may suffer accelerated wear and bit life can be too short to be commercially acceptable.
Studies have suggested that the rapid wear of PDC bits in harder formations is due to chipping of the cutters as a result of impact loads caused by vibration. This may be torsional vibration of the drill string, for example as a result of the natural frequency of torsional vibration of the drill string being excited by forces generated by the bottom hole assembly and, in particular, by the drill bit.
Torsional vibration can have the effect that cutters on the drill bit may momentarily be rotating backwards, i.e. in the reverse rotational direction to the normal forward direction of rotation of the drill bit during drilling. It is believed that it is this backward rotation which may be causing excessive damage to the cutters of PDC bits when drilling harder formations where torsional vibration is more likely to occur. The effect of reverse rotation on a PDC cutter may be to impose unusual loads on the cutter which tend to cause spalling or delamination, i.e. separation of part or all of the polycrystalline diamond facing table from the tungsten carbide substrate.
The present invention sets out to overcome this phenomenon by providing in the drill string a torque transmitting device which reduces the likelihood of reverse rotation of the cutters occurring. This is effected by damping out the torsional vibrations to such an extent that any momentary backward rotation of the drill bit is overcome by the forward rotational speed. Also, means are provided to prevent momentary backward rotation of the drill string being transmitted to the drill bit itself.